In a small bowl, add cold water. Sprinkle gelatin powder on top of the water and set it aside undisturbed for at least 5 minutes until the gelatin blooms.
Line a 9" springform pan with a strip of parchment paper or clear acetate around the side walls. Tape it at the seam to hold it in place.
Cut about ¼" off one end of the ladyfingers that will be going around the sides of the pan so that they have a flat bottom. Arrange them vertically around the sides of the pan with the sugared side facing outwards. Arrange a layer of ladyfingers on the bottom of the pan, cutting the biscuits to fit, if needed. Set aside.
In a large mixing bowl, add whipping cream, powdered sugar, and vanilla extract. Use an electric hand mixer to whip it on medium speed until it becomes firm peaks. Set aside.
To a blender, add the mango chunks. Pop your bowl of bloomed gelatin in the microwave for about 15 seconds until it melts into a clear liquid. Pour this into the blender and blend until the mangoes are pureed and smooth.
Immediately add the mango puree to the bowl of whipped cream and fold together with a spatula until no streaks remain.
Pour half of this mango mixture into your prepared pan on top of the ladyfingers. Smooth the top with a spatula.
Arrange another layer of ladyfingers on top of the mango mousse layer, cutting the biscuits to fit, if needed. Pour the rest of the mango mousse mixture on top. Smooth the top with a spatula.
Chill the cake in the fridge while you prepare the whipped cream topping.
Topping
In a large mixing bowl, add whipping cream, powdered sugar, and vanilla extract. Use an electric mixer to whip until it becomes firm peaks.
Transfer the whipped cream to a piping bag fitted with a large round tip (or just snip off the tip of the bag). Pipe the cream on top of the mango mousse. Top the cake with mango chunks.
Cover and chill the cake in the fridge for at least 4 hours or preferably overnight until the mousse is set before serving. TIP: For very clean slices, freeze the cake for about 30 minutes right before slicing.